The United Nations has called for urgent action to be taken to halt the spread of HIV.

It has warned that in many countries one in three teenagers risk dying prematurely from Aids because of failure to halt the spread of the disease.

The UN body responsible for fighting the disease, Unaids, criticised governments for failing to respond to the threat of Aids.

Its executive director, Peter Piot, has warned that the populations of many developing countries could be decimated unless the spread of Aids is reduced.

Dr Piot criticised the current approach to the disease saying it was "just a fraction" of what it should be.

"The Aids toll in hard-hit countries is altering the economic and social fabric of society.

"HIV will kill more than one-third of the young adults of countries where it has its firmest hold, yet the global response is still just a fraction of what it could be.

The Aids toll in hard-hit countries is altering the economic and social fabric of society

Dr Peter Piot, Unaids

"We need to respond to this crisis on a massively different scale from what has been done so far. "

Figures from Unaids show that in countries such as South Africa and Zimbabwe, where up to a quarter of the adult population are currently affected, Aids is set to claim the lives of around half of all 15-year-olds.

In Botswana, where about one in three adults are already HIV positive - the highest rate in the world - at least two-thirds of today's 15-year-old boys will die prematurely from Aids.

The figures are contained in a UN report which shows that current trends in HIV infection will increasingly have an impact on infant, child and adult mortality. It will also adversely affect life expectancy rates and economic growth prospects.

It is estimated that more than 24m adults and children in sub-Saharan Africa are living with HIV.

The report also warns that "risky" sexual behaviour is on the increase in the developed world, following a fall in Aids-related deaths and improvements in drug treatments.

In San Francisco, the proportion of gay men reporting multiple partners and unprotected anal sex rose between 1994 and 1998.

Success

But the UN also highlights some successes in the fight against Aids. In Zambia, the proportion of pregnant teenagers who have been infected with HIV has fallen by almost a half over the past six years.

In Uganda, the proportion of the total population with HIV has fallen to 8% from a high of 14% in the early 1990s.

"Achievements like these keep hope alive by proving that the world is not powerless against the epidemic," said Dr Piot.

"But up to now the gains have been scattered, not systematic. We need an all-out effort to turn the tide of the epidemic everywhere, with a massive increase in resources from domestic budgets and international development assistance."

Mohga Smith, health policy adviser with Oxfam, said part of the problem lay in the fact that some government's equated education with prevention.

"They believe that education is the same as prevention. But it is not. We have to get them to develop innovative ways of encouraging children to protect themselves against the disease."